Optical fibers have brought about breakthroughs in the globalization of communications to make high-quality and large-capacity inter-oceanic telecommunications feasible. So far, it has been known that a Bragg diffraction grating is provided in an optical fiber by creating a periodic refractive index profile in an optical fiber core along the optical fiber, and the magnitude of reflectivity and the width of the wavelength characteristics of the diffraction grating are determined by the period and length and the magnitude of refractive index modulation of the diffraction grating, whereby the diffraction grating can be used as a wavelength division multiplexer for optical communications, a narrow-band yet high-reflection mirror used for lasers or sensors, a wavelength selection filter for removing extra laser wavelengths in fiber amplifiers, etc.
However, the wavelength at which the attenuation of a quartz optical fiber is minimized and which is suitable for long-distance communications is 1.55 .mu.m. It is thus required hat the grating spacing be about 500 nm in order to allow the optical fiber diffraction grating to be used at this wavelength. At the beginning, it was considered difficult to make such a minute structure in an optical fiber core; that is, a Bragg diffraction grating was provided in the optical fiber core by a sophisticated process comprising a number of steps, e.g., side polishing, photoresist coating, holographic exposure, and reactive ion beam etching. For this reason, much fabrication time was needed, resulting in low yields.
In recent years, however, a method of fabricating a diffraction grating by irradiating an optical fiber with ultraviolet radiation to cause a refractive index change directly in an optical fiber core has been developed. This ultraviolet irradiation method has been steadily put to practical use with the advance of peripheral technologies, because of no need of any sophisticated processes.
Since the grating spacing is as fine as about 500 nm as mentioned above, this method using ultraviolet light is now carried out by a two-beam interference process, a writing-per-point process wherein single pulses from an excimer laser are focused to make diffraction grating surfaces one by one, an irradiation process using a phase mask having a grating, etc.
Regarding the two-beam interference process, a problem arises in conjunction with the quality of the beams in the lateral direction, i.e., spatial coherence. A problem with the writing-per-point process is on the other hand that strict step control of the submicron order is needed to focus light on a small point for writing light on many surfaces. Another problem arises in conjunction with processability.
To solve these problems, attention has focused on the irradiation process using a phase mask. According to this process, a phase mask 21 comprising a quartz substrate provided on one surface with grooves of given depth at a given pitch is irradiated with KrF excimer laser light (of 248 nm wavelength) 23 to give a refractive index change to a core 22A of an optical fiber 22, thereby producing a grating (diffraction grating), as shown in FIG. 7(a). For a better understanding of an interference pattern 24 on the core 22A, the pattern 24 is exaggerated in FIG. 7(a). FIG. 7(b) is a sectional view of the phase mask 21, and FIG. 7(c) is a partial top view corresponding to FIG. 7(b). The phase mask 21 has a binary phase type of diffraction grating structure where the substrate is provided on one surface with grooves 26 having a depth D at a repetition pitch P, with a strip 27 substantially equal in width to each groove being provided between adjacent grooves 26.
The depth of each groove 26 on the phase mask 21 (the difference in height between strip 27 and groove 26) D is chosen such that the phase of the excimer laser light (beam) 23 that is exposure light is modulated by .pi. radian. Thus, zero-order light (beam) 25A is reduced to 5% or less by the phase mask 21, and chief light (beam) leaving the mask 21 is divided into + first-order diffracted light 25B containing at least 35% of diffracted light and - first-order diffracted light 25C, so that the optical fiber 22 is irradiated with the + first-order diffracted light 25B and - first-order diffracted light 25C to produce an interference fringe at a given pitch, thereby providing a refractive index change at this pitch in the optical fiber 22.
When the diffracting grating is fabricated in the optical fiber 22 by interference of + first-order light 25B and - first-order light 25C using such a phase mask 21, deposition of foreign matters on the surface of the phase mask 21 causes defects in the diffraction grating exposed to light in the optical fiber 22. This in turn gives rise to noises in the characteristic spectra of the diffraction grating.
When the optical fiber 22 is irradiated with ultraviolet radiation 25B and 25C according to such an arrangement as shown in FIG. 7(a), the covering resin of the optical fiber 22 is sublimated due to exposure to ultraviolet radiation 25B and 25C, filling up the grooves 26 in the phase mask 21. This offers a similar defect problem with respect to the diffraction grating exposed to light in the optical fiber.
A prior grating constituting such a diffraction grating-fabricating phase mask 21 has a reduced diffraction efficiency and so shows an about 3% transmittance with respect to zero-order light 25A because the grooves 26 are of a rectangular wave shape in section, as shown in FIG. 7(b). This zero-order light component 25A makes noises, which in turn appear in the reflection spectra of the transferred optical waveguide diffraction grating.